Hi Robert,
Thanks so much for your response. As I mentioned, I am still very new to this customization process so it turned out to be a rookie mistake. After looking at the web console I was able to see that the app was trying to reference the wrong manifest URL for the widget, since IT changed the directory structure on our web server. (the reason I didn't suspect this initially is because the widget was displaying and behaving normally in the "builder" view). I was able to correct the situation by re-adding the widget as an Application Extension and updating the URL.
As a side-note, a huge thank you for all the work that you've contributed to this community. The widget I was adding was the "Popup Panel" widget you created, and it's extremely useful. Resources like this are a huge help to someone like me who is just starting to explore the possibilities of customization, coming from a strictly "ArcGIS Online/Esri Hosted Services" environment.
Cheers,
Katherine
Best,
Katie
“The goal is not simply to ‘work hard, play hard.’ The goal is to make our work and our play indistinguishable.”
- Simon Sinek